A variety of colour changers have been developed to control the colour of light transmitted from a source. Such devices are used widely to provide theatrical, decorative and architectural lighting.
Conventional colour changers make use of dichroic filters to vary the colour of light transmitted from a source. Dichroic filters reflect selected wavelengths while transmitting other wavelengths thereby allowing one to produce a desired colour. For example, U.S. Pat. No. 4,392,187, issued Jul. 5, 1983 to Bornhorst, discloses a system for varying the transmitted colour from a light source using four aligned dichroic filters within the path of the light beam. Each filter is pivotable so as to vary the angle of incidence of the light falling upon the filter, thereby varying the wavelength of light transmitted. In U.S. Pat. No. 4,602,321, issued Jul. 22, 1986 to Bornhorst, sets of overlapping dichroic filters are aligned in a light source. Each filter is rotatable around an axis so as to vary the angle of incidence which, in turn varies the hue and saturation of the filtered light beam, thereby producing a range of colours with a limited number of filters.
While the prior art systems provide useful mechanisms for varying the colour of light produced with a light source, they are not well suited for use with wide angle light sources, such as with floodlights. When a flood light or other light source with a wide area of coverage is used with a dichroic filter, the angle of incidence of light falling on the filter varies across the face of the filter. It is well known that wavelength of light transmitted by a dichroic filter is a function of the angle of the incident beam to the normal at the point of incidence. As discussed above, prior art colour changers make use of this principle to produce a variety of colours using only a few filters. In such conventional devices, however, the area provided by the filters for receiving a light beam is comparatively small and the rays of light from the source must be directed towards the filters in an orientation substantially parallel with a longitudinal axis of the light source. This arrangement is not suitable for wide angle coverage.
In order to produce a desired distribution of light conventional devices rely on a system of lenses positioned after the filters and in the path of the filtered beam. However, when designing a light source with a wide area of coverage, it is desirable to avoid the complexity and cost of lenses and to rely entirely on a reflector that produces the required distribution of light. A number of such wide coverage light sources are commonly made and come in a variety of shapes. As these light sources do no use lenses, colour filters for such sources must cover a large area equal at least to the size of the face of the reflector. Moreover, as wide coverage light sources are very bright, a non-absorbing dichroic filter is necessary to provide a low maintenance colour medium.
Accordingly, it is an object of the invention to provide an improved colour changer which can be used with a light source to cover a large area and provide at the same time improved uniformity of colour.